The success of positron imaging in expanding the role of nuclear medicine has lead to greater demands for resolution, sensitivity and flexibility. On the basis of nearly three decades of experience in positron instrumentation, we have designed and constructed an analog ring system for positron tomography (PC Ring I) that we believe will be the standard for such instruments for many years. This system will provide high resolution and sensitivity and of even greater importance provide data for tomograms to be obtained without interpolative motion. This permits a high degree of flexibility in dynamic observation and in applications such as cardiac gating.